Conventionally, an internal combustion engine adopts an exhaust emission control system such as an oxidation catalyst, nitrogen oxide catalyst, or a three way catalyst, for removing airborne pollutants in an exhaust gas of the engine as a measure for environmental protection. Further, it is a current issue to eliminate particulate matters such as soot and soluble organic fraction (SOF) in the exhaust gas of a diesel engine. Therefore, the exhaust emission control system for the engine has a particulate filter along an exhaust line of the engine. The particulate filter retains the particulate matters by passing the exhaust gas through a porous barrier to accumulate the particulate matters in the exhaust gas thereon.
A large amount of the particulate matters in the particulate filter increases a flow resistance of the exhaust gas and a backpressure of the engine and decreases a power of the engine. Thus, it is necessary to regenerate the particulate filter by burning the particulate matters therein regularly.
Particulate accumulations increase the differential pressure in the particulate filter. Japanese patent document JP-07-332065-A discloses a method for determining a schedule to clean the particulate filter when a differential pressure in the particulate filter exceeds a predetermined value. It is desirable to determine the schedule with both the differential pressure and an exhaust flow rate because the differential pressure also depends on the exhaust flow rate.
However, both the exhaust flow rate and the differential pressure fluctuate making it difficult to accurately estimate the amount of the particulate matters in the particulate filter.
Providing a low estimate of the amount of particulate may cause an increase in backpressure of the engine and a decrease in engine power, as described above. Furthermore, the rapid burning of the large amount of the particulate matters deteriorates the particulate filter fast. Providing a high estimate of the amount increases the time of burning the particulate matters and wears the particulate filter fast, thereby causing it to be replaced.
A sensor system for estimating the particulate amount with high accuracy increases a manufacturing cost.